Electrophoretic displays (also known as EPDs, electrophoretic image displays or EPIDs or EPID cells) are non-emissive devices based on the electrophoresis phenomenon influencing charged pigment particles suspended in a colored dielectric solvent. This type of display was first proposed in 1969. An EPD typically comprises a pair of opposed, spaced-apart plate-like electrodes, with spacers predetermining a certain distance between the electrodes. At least one of the electrodes, typically on the viewing side, is transparent. For the passive type of EPDs, row and column electrodes on the top (the viewing side) and bottom plates respectively, are needed to drive the displays. In contrast, an array of thin film transistors (TFTs) on the bottom plate and a common, non-patterned transparent conductor plate on the top viewing substrate are required for the active type EPDs. An electrophoretic fluid composed of a colored dielectric solvent and charged pigment particles dispersed therein is enclosed between the two electrodes.
When a voltage difference is imposed between the two electrodes, the pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus, the color showing at the transparent plate, determined by selectively charging the plates, can be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color. Intermediate color density (or shades of gray) due to intermediate pigment density at the transparent plate may be obtained by controlling the plate charge through a range of voltages or pulsing time.
To view a reflective EPD, an external light source is needed. For applications to be viewed in the dark, either a backlight system or a front pilot light system may be used. A transflective EPD equipped with a backlight system is typically preferred over a reflective EPD with a front pilot light because of cosmetic and uniformity reasons. However, the presence of light scattering particles in typical EPD cells greatly reduces the efficiency of the backlight system. A high contrast ratio in both bright and dark environments, therefore, is difficult to achieve for traditional EPDs.
A transmissive EPD is disclosed in U.S. Pat. No. 6,184,856 in which a backlight, color filters and substrates with two transparent electrodes are used. The electrophoretic cells serve as a light valve. In the collected state, the particles are positioned to minimize the coverage of the horizontal area of the cell and allow the backlight to pass through the cell. In the distributed state, the particles are positioned to cover the horizontal area of the pixel and scatter or absorb the backlight. However, the backlight and color filter used in this device consume a great deal of power and therefore are not desirable for hand-held devices such as PDAs (personal digital assistants) and e-books.
EPDs of different pixel or cell structures have been reported previously, for example, the partition-type EPD (M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148-1152 (1979)) and the microencapsulated EPD (U.S. Pat. Nos. 5,961,804 and 5,930,026). However, both types have their own problems as noted below.
In the partition-type EPD, there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movement of the particles such as sedimentation. However, difficulties are encountered in the formation of the partitions, the process of filling the display with an electrophoretic fluid, enclosing the fluid in the display and keeping the fluids of different colors separated from each other.
The microencapsulated EPD has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a dispersion of charged pigment particles that visually contrast with the dielectric solvent. The microcapsules are typically prepared in an aqueous solution, and to achieve a useful contrast ratio, their mean particle size is relatively large (50-150 microns). The large microcapsule size results in poor scratch resistance and a slow response time for a given voltage because a large gap between the two opposite electrodes is required for large capsules. Also, the hydrophilic shell of microcapsules prepared in an aqueous solution typically results in sensitivity to high moisture and temperature conditions. If the microcapsules are embedded in a large quantity of a polymer matrix to obviate these shortcomings, the use of the matrix results in an even slower response time and/or a lower contrast ratio. To improve the switching rate, a charge-controlling agent is often needed in this type of EPDs. However, the microencapsulation process in an aqueous solution imposes a limitation on the type of charge-controlling agents that can be used. Other drawbacks associated with the microcapsules system include poor resolution and poor addressability for color applications.
An improved EPD technology was disclosed in U.S. Pat. No. 6,930,818 (corresponding to WO 01/67170), U.S. Pat. No. 6,672,921 (corresponding to WO02/01281) and U.S. Pat. No. 6,933,098 (corresponding to WO02/65215); all of which are incorporated herein by reference. The improved EPD comprises isolated cells formed from microcups of well-defined shape, size and aspect ratio and filled with charged pigment particles dispersed in a dielectric solvent, preferably a fluorinated solvent. The filled cells are individually sealed with a polymeric sealing layer, preferably formed from a composition comprising a material selected from the group consisting of thermoplastics, thermosets and precursors thereof.
The microcup structure enables a format flexible and efficient roll-to-roll continuous manufacturing process for the EPDs. The displays can be prepared on a continuous web of a conductor film such as ITO/PET by, for example, (1) coating a radiation curable composition onto the ITO/PET film, (2) forming the microcup structure by a microembossing or photolithographic method, (3) filling the microcups with an electrophoretic fluid and sealing the microcups, (4) laminating the sealed microcups with the other conductor film and (5) slicing and cutting the display into a desirable size or format for assembling.
One advantage of this EPD design is that the microcup walls are in fact built-in spacers to keep the top and bottom substrates apart at a fixed distance. The mechanical properties and structural integrity of this type of displays are significantly better than other displays including those manufactured by using spacer particles. In addition, displays involving microcups have desirable mechanical properties including reliable display performance when the display is bent, rolled or under compression pressure from, for example, a touch screen application. The use of the microcup technology also eliminates the need of an edge seal adhesive, which would limit and predefine the size of the display panel and confine the display fluid inside a predefined area. The display fluid within a conventional display prepared by the edge sealing adhesive method will leak out completely if the display is cut in any way, or if a hole is drilled through the display. The damaged display will be no longer functional. In contrast, the display fluid within the display prepared by the microcup technology is enclosed and isolated in each cell. The microcup display may be cut into almost any dimensions without the risk of damaging the display performance due to the loss of display fluid in the active areas. In other words, the microcup structure enables a format flexible display manufacturing process, wherein the process produces a continuous output of displays in a large sheet format which can be cut into any desired sizes. The isolated microcup or cell structure is particularly important when cells are filled with fluids of different specific properties such as colors and switching rates. Without the microcup structure, it will be very difficult to prevent the fluids in adjacent areas from intermixing or being subject to crosstalk during operation.
For applications to be viewed in dark environments, the microcup structure effectively allows the backlight to reach the viewer through the microcup walls. Unlike traditional EPDs, even a low intensity backlight is sufficient for users to view in the dark the transflective EPDs based on the microcup technology. A dyed or pigmented microcup wall may be used to enhance the contrast ratio and optimize the intensity of backlight transmitted through the microcup EPDs. A photocell sensor to modulate the backlight intensity might also be used to further reduce the power consumption of such EPDs.
The microcup EPDs may have the traditional up/down switching mode, the in-plane switching mode or the dual switching mode. In the display having the traditional up/down switching mode or the dual switching mode, there are a top transparent electrode plate, a bottom electrode plate and a plurality of isolated cells enclosed between the two electrode plates. In the display having the in-plane switching mode, the cells are sandwiched between a top transparent insulator layer and a bottom electrode plate.
The electrophoretic dispersions may be prepared according to methods well known in the art, such as U.S. Pat. Nos. 6,017,584, 5,914,806, 5,573,711, 5,403,518, 5,380,362, 4,680,103, 4,285,801, 4,093,534, 4,071,430, and 3,668,106. See also IEEE Trans. Electron Devices, ED-24, 827 (1977), and J. Appl. Phys. 49(9), 4820 (1978).
The charged primary color particles are usually white and may be organic or inorganic pigments, such as TiO2. The particles may also be colored. The particles should have acceptable optical characteristics, should not be swollen or softened by the dielectric solvent and should be chemically stable.
Suitable charged pigment dispersions may be manufactured by grinding, milling, attriting, microfluidizing and ultrasonic techniques. For example, pigment particles in the form of a fine powder may be added to a suitable dielectric solvent and the resulting mixture is ball milled or attrited for several hours to break up the highly agglomerated dry pigment powder into primary particles.
U.S. Pat. No. 4,285,801, issued to A. Chiang, discloses a stable suspension for use in EPDs which suspension has high electrophoretic sensitivity. The high sensitivity was achieved by adsorbing highly fluorinated polymers onto the surface of the suspended pigment particles. It was determined that the fluorinated polymer shells were excellent dispersants as well as highly effective charge control agents. However, the adsorbed fluorinated polymer shell may become separated from the pigment particles during the operation of the display, causing destabilization of the pigment particles. Moreover, a common problem associated with this type of electrophoretic dispersions is sedimentation or creaming of the pigment particles particularly when high density pigment particles are used.
One method for achieving gravitational stability against sedimentation or creaming is to carefully select pigment and suspending liquid having similar or same specific gravities. However, when a dense inorganic pigment such as TiO2 (specific gravity ˜4) is employed, it is very difficult to find an organic solvent to match its density. This problem may be eliminated or alleviated by microencapsulating or coating the particles with a suitable polymer to match the specific gravity to that of the dielectric solvent.
Stabilization of pigment particles for use in EPDs has been effected by covalently bonding the pigment to a polymeric stabilizer. U.S. Pat. No. 5,914,806 discloses that charged pigment particles are substantially stabilized against agglomeration using polymeric stabilizers covalently bonded to the particle surface. The particles are organic pigments and the stabilizers are polymers with functional end groups capable of forming covalent bonds with the complementary functional groups of the organic pigment on the surface. Since only a thin layer of polymer is coated onto the pigment particles, it is very difficult, if not impossible, to match the specific gravity of dense particles, such as TiO2, to that of most commonly used organic solvents, by using this method.
Microencapsulation of the pigment particles may be accomplished either chemically or physically. Typical microencapsulation processes include interfacial polymerization, in-situ polymerization, phase separation, coacervation, electrostatic coating, spray drying, fluidized bed coating and solvent evaporation. Well-known procedures for microencapsulation have been disclosed in Kondo, Microcapsule Processing and Technology, Microencapsulation, Processes and Applications, (I.E. Vandegaer, ed.), Plenum Press, New York, N.Y. (1974), and Gutcho, Microcapsules and Microencapsulation Techniques, Nuyes Data Corp., Park Ridge, N.J. (1976), both of which are hereby incorporated by reference.
A process involving (1) dispersing pigment particles in a non-aqueous polymer solution, (2) emulsifying the dispersion in an aqueous solution containing surfactants, (3) removing the organic solvent and (4) separating the encapsulated particles, was disclosed in U.S. Pat. No. 4,891,245 for the preparation of specific gravity matched particles for use in EPD applications. However, the use of an aqueous solution in the process results in major problems such as flocculation caused by separation of the particles from water and undesirable environmental sensitivity of the display.
U.S. Pat. No. 4,298,448, issued to K. Muller and A. Zimmerman, discloses the application of particles of various pigments where the particles are coated with an organic material which is stable at the cell operating temperature but melts at higher temperatures. The organic coating material contains a charge control agent to impart a uniform surface potential which permits the particles to migrate in a controlled fashion.
Microencapsulation of pigment particles by interfacial polymerization/crosslinking can result in a highly crosslinked microcapsule that does not melt at an elevated temperature. If necessary, microcapsules may be post hardened by in-situ polymerization crosslinking reactions inside the microcapsules. However, typical dielectric solvents useful for EPD applications have a relatively low refractive index compared to most of crosslinked polymers. As a result, specific gravity matched pigment microcapsules having a thick layer of polymeric shell or matrix typically show a lower hiding power or lower light scattering efficiency than the non-capsulated pigment particles.
Therefore, there still exists a need for pigment particles with optimal characteristics for application in all type of EPDs, including traditional EPDs, microcup EPDs as well as encapsulated EPDs. Desirable particle characteristics include uniform size, surface charge, high electrophoretic mobility, stability against agglomeration, better shelf life stability, matching specific gravity with various dispersion fluids, better hiding power, lower Dmin, higher contrast ratio and other particle characteristics which provide for a wider latitude in the control of switching rate.